Preheat your oven to 200ºc/180ºc fan and line two/three baking trays with parchment paper.
Mix together the unsalted butter and two sugars until light and fluffy and thoroughly combined.
Add the vanilla and egg, and mix again briefly.
Add the cocoa powder, plain flour, bicarbonate of soda, baking powder and salt and mix until a thick paste/cookie dough is formed. I use my KitchenAid with the paddle attachment!
Chop up the Galaxy chocolate to 1cm(ish) size pieces, and fold through.
Split the mixture evenly into 9 cookies - I weighed it to make it accurate, and they were roughly 105g each. Put 3/4 on a tray each, as they will spread.
Put the cookie dough balls in the freezer for at least 30 minutes, or the fridge for 1 hour or so!
Bake in the oven for 11-13 minutes.
Once baked, remove from the oven and allow to cool slightly before gobbling up otherwise you might burn yourself on molten chocolate!
Notes
These are 'giant' size, so it only makes 9 in the recipe.
You can make them larger if you wish! I've made them as 150g each and only made 6, but they were very very large and only fit two on the tray.
I used Galaxy chocolate, but you can use Cadburys, or anything you fancy!
These will last for 4-5 days after baking, but they rarely do as they're eaten so quickly!